Most cars and the like have a temperature gauge to indicate engine coolant temperature and most of these are marked only with colors, typically blue for cold; yellow/orange/background for normal or safe range; and red for hot or danger. Unmarked and unnumbered scales are not uncommon. In use, most drivers check the temperature gauge very occasionally and as long as the indicating needle is not in the red zone assumes everything is fine in the cooling system. Often overheating of the engine occurs without being observed. However, more problematical and not uncommon is that engines can overheat due to cooling system faults that are not reflected by the gauge.
Modern vehicle engines depend upon a pressurised cooling system to raise the boiling point of engine coolant for increased combustion efficiency. Most modern cooling systems operate at a pressure of .+-.100 kPa at which pressure the boiling point is about 120.degree. C. The red line on the temperature gauge in this event would be marked at .+-.115.degree. C. The inventors have realized that anything which leads to loss of pressure in the cooling system from, say, a defective radiator pressure cap, low coolant level, leaking hoses, etc., will cause the boiling point of the coolant to drop to about 100.degree. C., ie. the boiling point of water at atmospheric pressure. Unfortunately, at this temperature the temperature gauge indicates well below the danger zone. Thus the driver proceeds, often at high speed, blissfully unaware that anything is amiss until the engine stops unexpectedly, because the engine has seized or the cylinder head has cracked and warped.